The coating of substrates by cathode sputtering in plasma processes is well known. The sputtering can be done conventionally. In some cases, the sputtering can be done by using a reactive gas, which is also called reactive sputtering. To that end, a power supply produces a plasma to remove material from a target, and the removed material is then coated on the substrate. If a reactive process is used, the target atoms in the removed material can combine with gas atoms or molecules, depending on the desired coating.
The electrical impedance of a plasma in a plasma chamber can vary. In particular, the impedance can vary instantly, for example, there may be a high impedance before the plasma ignites and a low impedance once the plasma has been ignited. If a pulsed power supply is used, the impedance of the plasma may change frequently due to frequent igniting of the plasma.
The power produced by an RF power source is supplied to a plasma chamber via an impedance matching unit. The impedance matching unit tries to match the impedance of the plasma to the output impedance of the RF power source.
Plasma processes can be powered by more than one RF power source. Furthermore, plasma parameters may change with time, and the plasma may appear or disappear. This is happening during ignition, arcing or for other reasons. Changing of the plasma properties results in a variation of the (plasma) load impedance an impedance matching unit sees and tries to match to. While the plasma properties change, the power sources and impedance matching units try to adapt to the plasma with the intention that the process result, in particular deposition, is stable in time. If the plasma is powered with many sources, the sources compete in stabilizing the plasma.
In some systems, the plasma impedance matching units each tried to match the impedance of the respective RF power source they were connected to with the impedance of the plasma, not knowing what the other impedance matching units of the plasma supply system were doing. There have been attempts to use a central controller controlling the plasma impedance matching units of a plasma power supply system. However, there was still no interaction between the plasma impedance matching units. Furthermore, the central controller had no information about the plasma state. Therefore, a very crude method had been suggested by performing impedance matching only with one impedance matching unit when the plasma is being ignited and then to use all the impedance matching units for further matching. There had been no provisions for a reaction to instabilities in the plasma.